The present invention relates to an electronic operating device for supplying lamps, which, for its part, is supplied with AC voltage.
This encompasses, on the one hand, operating devices for individual lamps but also, on the other hand, operating devices which can supply a plurality of lamps.
In many operating devices of this type, a storage capacitor has to be charged from the AC voltage and supplies downstream circuit sections with DC voltage. To that end, the AC voltage, usually oscillating between two polarities, can be rectified and applied as unipolar AC voltage to the capacitor. However, this has the serious disadvantage that the capacitor is charged from the AC voltage only when the instantaneous voltage value of the AC voltage is higher than the voltage across the capacitor. There arise, therefore, momentary pulsed charging surges in the vicinity of the voltage maxima of the AC supply voltage. These surge-like charging currents contain a regularly impermissibly strong harmonic spectrum. Special converter circuits are used for correction (so-called power factor correction (PFC)).
A known design of such a converter circuit is referred to as a step-up converter. One example is found in WO 96/03017. This solution has the advantage of good efficiencies and can be realized relatively inexpensively and simply. However, it has the disadvantage firstly that, in principle, it can only generate DC voltages which are greater than the AC supply voltage amplitude. Secondly, these converters generate relatively strong radio interference. What is critical is primarily the fundamentally governed lower limit for the DC voltage of the storage capacitor, which is highly disturbing with regard to the required dielectric strength of said capacitor and the downstream circuit sections that it supplies, and thus with regard to the circuit costs.
By contrast, in the case of a so-called SEPIC converter, the DC voltage with which the storage capacitor is charged can be set arbitrarily within relatively wide limits and, in particular, may also be smaller than the voltage amplitude of the AC supply voltage. On the other hand, this converter principle requires a number of additional components. In the case of the SEPIC converter, what is primarily of interest is the so-called intermittent operation in which the individual charging and discharging phases of the two inductances provided in said converter are separated by currentless times. The SEPIC converter is conventionally controlled by a fixed-frequency regulating circuit. The need for the desired current consumption waveform to be tracked by the duty ratio is obviated. Thus, the control circuit of the SEPIC converter is fundamentally simpler than in the case of the previously described step-up converter.
Both circuit concepts are familiar to the person skilled in the art and need not be explained in detail here. With regard to the functioning of the SEPIC converter, however, reference is supplementarily made to the following description of the invention.
The present invention is based on the technical problem of providing an improved electronic operating device for lamps with a SEPIC converter. According to the invention, it is provided for this purpose that the electronic operating device has a storage capacitor, a SEPIC converter for charging the storage capacitor from the AC voltage to a DC voltage in the case of an operating device current consumption which is essentially tracked to the AC voltage waveform with an operating frequency, and a regulating circuit for regulating the SEPIC converter to an output quantity, characterized in that the regulating circuit alters the operating frequency of the SEPIC converter during regulation.
Refinements of the invention emerge from the dependent claims.
The essential aspect of the invention resides in the SEPIC converter of the operating device not being controlled in a fixed-frequency manner, but rather the operating frequency thereof being altered, for the purpose of regulation, to an output quantity of the SEPIC converter as regulated variable. The output quantity might be for example the output voltage of the SEPIC converter, which charges the storage capacitor. In principle, however, a different output quantity of the converter or downstream circuit sections is also appropriate.
It has been conventional practice for the duty ratio to be altered during regulation, but the frequency to be kept fixed. By way of example, with a decreasing supply voltage amplitude (mains power supply voltage fluctuations), the switch-on time of the switching transistor of the SEPIC converter has been increased and the switch-off time decreased, the overall period having remained unchanged. It has thus been possible to obtain a constant output voltage despite mains power supply voltage fluctuations.
However, the inventors assume that the fixed-frequency driving of the switching transistor is highly unfavorable with regard to the line-conducted interference of the operating device. Therefore, according to the invention, the operating frequency of the switching transistor is to be altered during the regulation of the output quantity, so that the line-conducted interference is distributed over a certain frequency range in the time profile.
The switch-on time of the switching transistor of the SEPIC converter is preferably intended to be altered in this case. As a result, the regulation can thus be effected by way of an alteration of the switch-on time of the switching transistor and the resultant operating frequency alteration. Furthermore, the switch-off time of the switching transistor is again preferably intended to remain constant. This has the additional advantage that the switch-off time can be optimized in such a way as to ensure the intermittent operation which is preferred according to the invention. Thus, the switch-on time is preferably the (sole) manipulated variable.
The intermittent operation presupposes an, albeit short, currentless phase (in a diode in series with the storage capacitor) between the charging and discharging pulses of the inductances of the SEPIC converter. The discharging pulses belong to the switch-off time of the switching transistor. Since regulation to a constant output voltage is normally effected, the discharge behavior is not subjected to any significant fluctuations, so that the switch-off time can be set to a time-invariable optimum value in order to ensure the intermittent operation.
This optimum value should be chosen such that the switch-off time is as short as possible, but still ensures intermittent operation. In the most favorable case, the term mentioned here is quasi-transient operation which, according to the invention, is preferably chosen such that the currentless time at the end of the switch-off time before a new switch-on phase of the switching transistor amounts to at most 10% of the period duration, that is to say of the reciprocal operating frequency of the SEPIC converter.
On the other hand, the switch-on time of the switching transistor can be used to adapt the charging operation of the inductances in such a way that an essentially constant output quantity, in particular output voltage, can nevertheless be made available under varying charging conditions on account of mains power supply voltage fluctuations or similar interference.
In addition to avoiding the line-conducted interference in the case of a fixed-frequency control circuit, an optimization of the xe2x80x9cutilizationxe2x80x9d of the inductances is thus produced as a result of the optimization of the switch-off time. A maximum value depending on the lowest permissible supply voltage and the lamp can be chosen for the switch-on time.
The regulating circuit used is preferably digital and regulates the SEPIC converter in time-discrete fashion. In this case, the intervals between the regulating operations are preferably at least half a period of the AC supply voltage, in respect of which reference is made to the prior application xe2x80x9cOperating device for a lamp with a SEPIC converterxe2x80x9d of Jan. 24, 2001 from the same applicant. This application bears the file reference 101 10 239.9.
To put it briefly, this exploits the property of a SEPIC converter whereby a sufficiently good power factor can be achieved in intermittent operation in the case of a constant duty ratioxe2x80x94apart from the regulation to the output quantityxe2x80x94between switch-on time and switch-off time. In this case, then, the duty ratio is not tracked to the instantaneous mains power supply voltage, but rather is merely regulated according to the output quantity as regulated variable. Regulating intervals of a time-discrete regulation of at least half the mains power supply period are therefore sufficient. The AC supply voltage cannot be tracked with such regulating intervals. The advantage resides in the less stringent requirements made of the regulating circuit and the saving possibilities thus obtained.
A preferred application of the invention is to provide an electronic operating device for a plurality of lamp types. Said operating device has a programmable control circuit with a memory with operating parameters for said lamp types. In particular, the programmable control circuit also contains the regulating circuit for the control circuit of the SEPIC converter and can adapt the latter to the corresponding parameters of the lamp, that is to say set it for example to the desired output voltage taking account of the lamp impedance. The control circuit of the SEPIC converter can then set parameters adapted to the specific lamp type that is detected by the operating device or communicated to it at the beginning of operation, for instance the switch-off time, the range of selectable switch-on times and the frequency. In particular, it is preferred in this case for the operating device to automatically identify a newly connected lamp at the beginning of operation of said lamp.
Furthermore, it is preferred for the operating device to perform, at the beginning of operation, an automatic adjustment to the specific parameters in the context of customary component fluctuations prescribed by the mounted components. As a result, the requirements made of the component selection can be kept in a cost-effective framework and precise optimized operation can be ensured at the same time. This aspect applies, moreover, both to multitype operating devices and to operating devices designed only for one specific lamp type.
Finally, in many cases the operating device has an oscillator, which is preferably controlled by the same regulating circuit which also controls the SEPIC converter. In particular, the entire operating device can be controlled by a single digital IC.
The preferred area of application is for operating devices for low-pressure gas discharge lamps. The oscillators used in this case are preferably half-bridge oscillators with external control by the abovementioned IC.